General information
ReFRESCO is the viscous-flow CFD code used to compute the flow around propellers, open or ducted, pump-jets or thrusters. For open-water conditions, CFD calculations have significant added value when compared with traditional approaches: 1) modelling accuracy is higher than potential flow approaches; 2) provides substantially more insight in the physics than model-testing; 3) permits to check scale effects. The often referred drawback “it is too CPU-time expensive” is nowadays mitigated at MARIN by the use of smart numerical algorithms, steady calculations, and HPC clusters; a standard calculation for a propeller in open-water condition can be done in a couple of hours. When talking about propulsors (and propulsion), accuracy is a hot topic, since 1% of increase of efficiency may lead to significant amounts of savings (fuel and costs). Therefore numerical verification and validation of ReFRESCO for propeller flows is crucial, and is continuously done at MARIN since 2008.

Unstructured grid

Numerical Issues
Since very high accuracies are needed for the analysis of propulsors, small details do play a role. Therefore proper geometry definition, high quality grids and higher-order numerical schemes have to be used in order to reach low numerical uncertainties. On the geometry side, MARIN has now the tools to correctly define propeller tip region, leading and trailing edges, hub-blade root connections, fillets. And on the grid-generation side MARIN can now produce both structured grids around simple to moderately complex blade geometries, and unstructured grids for complex blade geometries and propulsor arrangements. Combination of both is also possible.

Example 1: open propeller
Numerical studies, numerical verification and validation have been done for the well-known INSEAN E779A propeller benchmark test-case. The pictures below illustrate (top-left) the velocity field at the slip-stream of the propeller, together with an iso-surface of the Q, which is a quantity used to identify vortical structures. The picture (top-right) illustrates the comparison between experimental data, PROCAL calculations and ReFRESCO calculations for the complete open-water advanced coefficient range.

Example 2: ducted propeller
For a ducted propeller, the flow is more complex due to the interaction of duct and blades (see picture below). Conventional approaches are difficult for these cases, and standard model-tests do not easily provide insight in the flow physics. After numerical verification and validation, scale effects have been studied using ReFRESCO. The different flow vorticity fields, for model scale and full scale for two different Js (left J=0.30, right J=1.0) can be visualized in the next picture.

Ducted-propeller scale-effects study.(left) J=0.3; (right) J=1.0

Most recent papers on CFD for propulsors presented by MARIN
(complete overview of publications can be found under Publications)